A certain enzyme uses Mn²⁺ as a cofactor under physiological conditions, and scientists hypothesize that Mn²⁺ is required for stabilization of tertiary structure but not for enzyme activity. Which of the following observations is consistent with this hypothesis?

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11ecba2d_0ec7_c3df_a3bf_f336e9f1820f_MD0008_00 The ability of a protein to maintain its three-dimensional folded form (tertiary structure) is known as conformational stability. Conformational stability can be measured by melting temperature T_m, the temperature at which half of the proteins in solution are folded and half are denatured. A lower T_m indicates decreased conformational stability because less thermal energy is required to denature the protein. The ability of a folded enzyme to catalyze a reaction is measured by the turnover number k_cat, which represents the number of reactions catalyzed per second per active site in solution. A change in the conformational stability of an enzyme does not necessarily affect function, as long as the ambient temperature is low enough that the protein can fold properly. In this scenario, scientists hypothesized that Mn²⁺ is required for stabilization of tertiary structure (ie, for conformational stability), but not for enzyme activity. If this hypothesis is correct, then when Mn²⁺ is removed or replaced by a different ion, the T_m of the protein will decrease due to decreased conformational stability. However, the k_cat will not change because the ability of folded proteins to catalyze the reaction has not changed. Therefore, substitution of a different metal ion such as Mg²⁺ should decrease T_m without affecting k_cat. (Choice B) If Cu²⁺ yielded the same T_m as Mn²⁺, it would indicate that Mn²⁺ is not required for conformational stability. A decreased k_cat would indicate that Mn²⁺ is important for catalysis. (Choice C) A decreased T_m indicates decreased conformational stability, but a k_cat of zero when Ni²⁺ replaces Mn²⁺ would indicate that Mn²⁺ is required for catalysis. (Choice D) An increased T_m in the absence of Mn²⁺ would indicate that Mn²⁺ is not required for conformational stability and actually reduces stability. Educational objective: The conformational stability of a protein is its ability to maintain its three-dimensional folded form and can be measured by the melting temperature (T_m) at which half of the proteins in solution are folded and half are denatured. Conformational stability does not necessarily affect protein function, as long as the protein is maintained at a temperature that allows proper folding.

A Certain Enzyme Uses Mn2+ as a Cofactor Under Physiological

A Certain Enzyme Uses Mn²⁺ as a Cofactor Under Physiological